Frame transmission method performed in access point, frame reception method performed in terminal, and access point

ABSTRACT

Provided are a frame transmission method performed in an access point (AP), a frame reception method performed in a terminal, and an AP. The frame transmission method performed in an AP includes acquiring a channel for transmitting a first data unit of a first terminal, when a data unit to be transmitted to at least one terminal other than the first terminal is in the AP, generating a physical layer convergence procedure (PLCP) protocol data unit (PPDU) including the first data unit and the data unit to be transmitted to the at least one terminal, and transmitting the PPDU to the first terminal and the at least one terminal. Accordingly, it is possible to improve the performance of a wireless local area network (WLAN).

CLAIM FOR PRIORITY

This application is a continuation of co-pending U.S. application Ser.No. 15/722,492, filed on Oct. 2, 2017, and allowed on Apr. 25, 2019,which is a continuation of U.S. application Ser. No. 14/577,219, filedon Dec. 19, 2014 (now U.S. Pat. No. 9,814,092, issued on Nov. 7, 2017).Further, these applications are based upon and claims the benefit ofpriority of the prior Korean Patent Application No. 2013-0160129 filedon Dec. 20, 2013 and No. 2014-0183716 filed on Dec. 19, 2014 in theKorean Intellectual Property Office (KIPO), the entire contents of whichare hereby incorporated by reference.

BACKGROUND 1. Technical Field

Example embodiments of the present invention relate in general towireless local area network (WLAN) technology, and more particularly, toa technology for transmitting and receiving a frame includingmultiplexed data units.

2. Related Art

With the development of information and communication technology, avariety of wireless communication technologies are under development.Among these, a WLAN is a technology for enabling wireless Internetaccess through a portable terminal, such as a personal digital assistant(PDA), a laptop computer, a portable multimedia player (PMP), a smartphone, or a tablet personal computer (PC), in a home, a company, or aspecific service providing area based on radio frequency technology.

Standards of a WLAN are being developed as the Institute of Electricaland Electronics Engineers (IEEE) 802.11 standards. A WLAN technologyconforming to the IEEE 802.11a standard operates based on orthogonalfrequency division multiplexing (OFDM), and may provide a maximumtransmission rate of 54 Mbps in the 5 GHz band. A WLAN technologyconforming to the IEEE 802.11b standard operates based on directsequence spread spectrum (DSSS), and may provide a maximum transmissionrate of 11 Mbps in the 2.4 GHz band. A WLAN technology conforming to theIEEE 802.11g standard operates based on OFDM or DSSS, and may provide amaximum transmission rate of 54 Mbps in the 2.4 GHz band.

A WLAN technology conforming to the IEEE 802.11n standard operates inthe 2.4 GHz band and the 5 GHz band based on OFDM, and may provide amaximum transmission rate of 300 Mbps to four spatial streams whenmultiple-input multiple-output OFDM (MIMO-OFDM) is used. The WLANtechnology conforming to the IEEE 802.11n standard may support a channelbandwidth up to 40 MHz, and in this case, it is possible to provide amaximum transmission rate of 600 Mbps.

The prevalence of such a WLAN and the diversification of applicationsusing a WLAN are leading to an increase in the necessity for a new WLANtechnology for supporting a higher throughput than the data processingspeed supported by IEEE 802.11n. A very high throughput (VHT) WLANtechnology is one of the IEEE 802.11 WLAN technologies that have beenproposed to support a data processing speed of 1 Gbps or more. Amongthem, IEEE 802.11ac is under development as a standard for providing aVHT in a band of 5 GHz or less, and IEEE 802.11ad is under developmentas a standard for providing a VHT in a 60 GHz band.

Recently, the number of stations (STAs) associated to a basic serviceset (BSS) of a WLAN has been drastically increasing, and collisionsbetween STAs in a BSS have been increasing accordingly. In thissituation, a transmission and reception method for efficientlyexchanging data between STAs is necessary.

SUMMARY

Accordingly, example embodiments of the present invention are providedto substantially obviate one or more problems due to limitations anddisadvantages of the related art.

Example embodiments of the present invention provide a method oftransmitting and receiving multiplexed data.

Example embodiments of the present invention also provide an apparatusfor transmitting and receiving multiplexed data.

In some example embodiments, a frame transmission method performed in anaccess point (AP) includes: acquiring a channel for transmitting a firstdata unit of a first terminal; when a data unit to be transmitted to atleast one terminal other than the first terminal is in the AP,generating a physical layer convergence procedure (PLCP) protocol dataunit (PPDU) including the first data unit and the data unit to betransmitted to the at least one terminal; and transmitting the PPDU tothe first terminal and the at least one terminal.

Here, the PPDU may further include at least one of the lengths of thedata units, receiver identifiers (IDs) of the data units, channels inwhich the data units are transmitted, transmission sections, andtransmission spaces.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a frequency domain in the PPDU.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a time domain in the PPDU.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a spatial domain in the PPDU.

Here, the data units may be media access control (MAC) protocol dataunits (MPDUs) or MAC service data units (MSDUs).

Here, the frame transmission method may further include receivingacknowledgement (ACK) frames which are responses to reception of thePPDU from the first terminal and the at least one terminal.

In other example embodiments, a frame reception method performed in aterminal includes: receiving a PPDU obtained by multiplexing a pluralityof data units from an AP; determining whether a first data unit of theterminal is in the PPDU through a signal field included in the PPDU; andwhen the first data unit is in the PPDU, receiving the first data unitthrough resources indicated by the signal field.

Here, the signal field may include at least one of the lengths of theplurality of data units, receiver IDs of the plurality of data units,channels in which the plurality of data units are transmitted,transmission sections, and transmission spaces.

Here, the plurality of data units may be multiplexed in a frequencydomain in the PPDU.

Here, the plurality of data units may be multiplexed in a time domain inthe PPDU.

Here, the plurality of data units may be multiplexed in a spatial domainin the PPDU.

Here, the frame reception method may further include transmitting an ACKframe which is a response to reception of the first data unit to the AP.

In other example embodiments, an AP includes: a processor; and a memoryconfigured to store at least one command executed by the processor. Theat least one command is executable to perform operations of: acquiring achannel for transmitting a first data unit of a first terminal; when adata unit to be transmitted to at least one terminal other than thefirst terminal is in the AP, generating a PPDU including the first dataunit and the data unit to be transmitted to the at least one terminal;and transmitting the PPDU to the first terminal and the at least oneterminal.

Here, the PPDU may further include at least one of the lengths of thedata units, receiver IDs of the data units, channels in which the dataunits are transmitted, transmission sections, and transmission spaces.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a frequency domain in the PPDU.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a time domain in the PPDU.

Here, the first data unit and the data unit to be transmitted to the atleast one terminal may be multiplexed in a spatial domain in the PPDU.

Here, the data units may be MPDUs or MSDUs.

Here, the at least one command may be executed to further perform anoperation of receiving ACK frames which are responses to reception ofthe PPDU from the first terminal and the at least one terminal.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments of the present invention will become more apparentby describing in detail example embodiments of the present inventionwith reference to the accompanying drawings, in which:

FIG. 1 is a block diagram showing an example embodiment of a station(STA) that performs methods according to the present invention;

FIG. 2 is a conceptual diagram showing an example embodiment of theconfiguration of a wireless local area network (WLAN) system conformingto the Institute of Electrical and Electronics Engineers (IEEE) 802.11;

FIG. 3 is a conceptual diagram illustrating an example embodiment of amethod of transmitting and receiving a frame in a WLAN;

FIG. 4 is a block diagram of a physical layer convergence procedure(PLCP) protocol data unit (PPDU) including an aggregate-media accesscontrol (MAC) protocol data unit (A-MPDU);

FIG. 5 is a block diagram of a PPDU including an aggregate-MAC servicedata unit (A-MSDU);

FIG. 6 is a conceptual diagram illustrating another example embodimentof a method of transmitting and receiving a frame in a WLAN;

FIG. 7 is a sequence diagram illustrating a method of transmitting andreceiving a frame according to an example embodiment of presentinvention;

FIG. 8 is a flowchart illustrating an operation of generating a PPDU ina method of transmitting and receiving a frame according to an exampleembodiment of present invention;

FIG. 9 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency domain;

FIG. 10 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency and time domains;

FIG. 11 is a block diagram of an example embodiment of a PPDU includingMPDUs multiplexed in the time domain;

FIG. 12 is a block diagram of an example embodiment of a PPDU includingMDSUs multiplexed in the time domain;

FIG. 13 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency, time, and spatial domains;

FIG. 14 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency domain;

FIG. 15 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency and time domains;

FIG. 16 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency, time, and spatial domains; and

FIG. 17 is a block diagram showing the configuration of an access point(AP) according to an example embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE PRESENT INVENTION

Example embodiments of the present invention are described below insufficient detail to enable those of ordinary skill in the art to embodyand practice the present invention. It is important to understand thatthe present invention may be embodied in many alternate forms and shouldnot be construed as limited to the example embodiments set forth herein.Accordingly, while the invention can be modified in various ways andtake on various alternative forms, specific embodiments thereof areshown in the drawings and described in detail below as examples. Thereis no intent to limit the invention to the particular forms disclosed.On the contrary, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theappended claims. Elements of the example embodiments are consistentlydenoted by the same reference numerals throughout the drawings anddetailed description.

It will be understood that, although the terms “first,” “second,” “A,”B,” etc. may be used herein in reference to elements of the invention,such elements should not be construed as being limited by these terms.For example, a first element could be termed a second element, and asecond element could be termed a first element, without departing fromthe scope of the present invention. Herein, the term “and/or” includesany and all combinations of one or more referents.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements. Other words used to describe relationships betweenelements should be interpreted in a like fashion (i.e., “between” versus“directly between,” “adjacent” versus “directly adjacent,” etc.).

The terminology used herein to describe embodiments of the invention isnot intended to limit the scope of the invention. The articles “a,”“an,” and “the” are singular in that they have a single referent,however the use of the singular form in the present document should notpreclude the presence of more than one referent. In other words,elements of the invention referred to in the singular may number one ormore, unless the context clearly indicates otherwise. It will be furtherunderstood that the terms “comprises,” “comprising,” “includes,” and/or“including,” when used herein, specify the presence of stated features,items, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features, items,steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein are to be interpreted as is customary in the art towhich this invention belongs. It will be further understood that termsin common usage should also be interpreted as is customary in therelevant art and not in an idealized or overly formal sense unlessexpressly so defined herein.

Hereinafter, example embodiments of the present invention will bedescribed in greater detail with reference to the accompanying drawings.In order to facilitate general understanding in describing the presentinvention, the same components in the drawings are denoted with the samereference signs, and repeated description thereof will be omitted.

Throughout this disclosure, a station (STA) denotes an arbitraryfunctional medium including an Institute of Electrical and ElectronicsEngineers (IEEE) 802.11-conformant media access control (MAC) andphysical layer (PHY) interface for a wireless medium. STAs may beclassifies as STAs that are access points (APs) and STAs that arenon-APs. A STA that is an AP may be simply referred to as an AP, and aSTA that is a non-AP may be simply referred to as a terminal.

A STA may include a processor and a transceiver, and may further includea user interface, a display device, and so on. The processor denotes aunit devised to generate a frame that will be transmitted via a wirelessnetwork or to process a frame received via the wireless network, and mayperform various functions for controlling the STA. The transceiver isfunctionally connected to the processor, and denotes a functional unitdevised to transmit and receive a frame for the STA via the wirelessnetwork.

An AP may be referred to as a centralized controller, a base station(BS), a radio access station, a node B, an evolved node B, a relay, amobile multihop relay (MMR)-BS, a base transceiver system (BTS), a sitecontroller, etc., and may include all or a part of functions thereof.

A terminal (i.e., non-AP) may be referred to as a wirelesstransmit/receive unit (WTRU), user equipment (UE), a user terminal (UT),an access terminal (AT), a mobile station (MS), a mobile terminal, asubscriber unit, a subscriber station (SS), a wireless device, a mobilesubscriber unit, etc., and may include all or some of functions thereof.

Here, the terminal may denote a desktop computer, a laptop computer, atablet personal computer (PC), a wireless phone, a mobile phone, a smartphone, a smart watch, smart glasses, an e-book reader, a portablemultimedia player (PMP), a portable game machine, a navigation device, adigital camera, a digital multimedia broadcasting (DMB) player, adigital audio recorder, a digital audio player, a digital picturerecorder, a digital picture player, a digital video recorder, a digitalvideo player, etc. capable of communicating.

FIG. 1 is a block diagram showing an example embodiment of a STA thatperforms methods according to the present invention.

Referring to FIG. 1, a STA 100 may include at least one processor 110, amemory 120, and a network interface device 130 connected to a network toperform communication. Also, the STA 100 may further include an inputinterface device 140, an output interface device 150, a storage device160, and so on. Respective components included in the STA 100 may beconnected by a bus 170 and communicate with each other.

The processor 110 may execute program commands stored in the memory 120and/or the storage device 160. The processor 110 may denote a centralprocessing unit (CPU), a graphics processing unit (GPU), or a processordedicated to performing methods according to example embodiments of thepresent invention. The memory 120 and the storage device 160 may bevolatile storage media and/or non-volatile media. For example, thememory 120 may be a read only memory (ROM) and/or a random access memory(RAM).

Example embodiments of the present invention are applied to wirelesslocal area network (WLAN) systems conforming to IEEE 802.11, and mayalso be applied to communication systems other than the WLAN systemsconforming to IEEE 802.11.

For example, the embodiments of the present invention may be applied toa mobile Internet, such as a wireless personal area network (WPAN), awireless body area network (WBAN), a wireless broadband Internet(WiBro), or a world interoperability for microwave access (WiMax), asecond generation (2G) mobile communication network, such as globalsystem for mobile communication (GSM) or code division multiple access(CDMA), a 3G mobile communication network, such as wideband codedivision multiple access (WCDMA) or CDMA2000, a 3.5G mobilecommunication network, such as high speed downlink packet access (HSDPA)or high speed uplink packet access (HSUPA), a 4G mobile communicationnetwork, such as long term evolution (LTE) or LTE-Advanced, a 5G mobilecommunication network, and so on.

FIG. 2 is a conceptual diagram showing an example embodiment of theconfiguration of a WLAN system conforming to IEEE 802.11.

Referring to FIG. 2, a WLAN system conforming to IEEE 802.11 may includeat least one basic service set (BSS). The BSS denotes a set of STAsincluding STA1, STA2 (AP1), STA3, STA4, STA5 (AP2), STA6, STA7, and STA8that are successfully synchronized with each other to communicate witheach other, and is not a concept denoting a specific area.

BSSs may be classified as infrastructure BSSs and independent BSSs(IBSSs). Here, BSS1 and BSS2 are infrastructure BSSs, and BSS3 is anIBSS.

BSS1 may include a first terminal STA1, a first AP STA2 (AP1) thatprovides a distribution service, and a distribution system DS thatconnects the plurality of APs STA2 (AP1) and STA5 (AP2). In BSS1, thefirst AP STA2 (AP1) may manage the first terminal STA1.

BSS2 may include a second terminal STA3, a third terminal STA4, a secondAP STA5 (AP2) that provides a distribution service, and the distributionsystem DS that connects the plurality of APs STA2 (AP1) and STA5 (AP2).In BSS2, the second AP STA5 (AP2) may manage the second terminal STA3and the third terminal STA4.

BSS3 denotes an IBSS operating in an ad-hoc mode. In BSS3, there is notany AP that is a centralized management entity. In other words, in BSS3,the terminals STA6, STA7, and STA8 are managed in a distributed manner.In BSS3, all the terminals STA6, STA7, and STA8 may denote mobileterminals, and constitute a self-contained network because access to thedistribution system DS is not allowed.

The APs STA2 (AP1) and STA5 (AP2) may provide access to the distributionsystem DS for the terminals STA1, STA3, and STA4 connected theretothrough a wireless medium. In BSS1 and BSS2, communication between theterminals STA1, STA3, and STA4 is generally performed through the APsSTA2 (AP1) and STA5 (AP2). However, when a direct link is established,direct communication between the terminals STA1, STA3, and STA4 ispossible.

The plurality of infrastructure BSSs may be connected with each otherthrough the distribution system DS. The plurality of BSSs connectedthrough the distribution system DS is referred to as an extended serviceset (ESS). The entities STA1, STA2 (AP1), STA3, STA4, and STA5 (AP2)included in the ESS may communicate with each other. In the same ESS,arbitrary terminals STA1, STA3, and STA4 may move from one BSS toanother BSS while seamlessly communicating.

The distribution system DS is a mechanism for one AP to communicate withanother AP. According to the distribution system DS, an AP may transmita frame for terminals associated to a BSS managed by the AP, or transmita frame for an arbitrary terminal having moved to another BSS. Also, anAP may exchange frames with an external network, such as a wirednetwork. This distribution system DS is not necessarily a network andmay have any form as long as a predetermined distribution servicedefined in the IEEE 802.11 standards is provided. For example, thedistribution system DS may be a wireless network, such as a meshnetwork, or a physical structure that connects APs with each other.

FIG. 3 is a conceptual diagram illustrating an example embodiment of amethod of transmitting and receiving a frame in a WLAN.

Referring to FIG. 3, a first STA STA1 that intends to transmit data maydetermine whether or not a channel has been occupied based on clearchannel assessment (CCA) that is a standard WLAN sensing technique. Whena result of CCA indicates that the channel is in an idle state, if thechannel is in the idle state within a distributed coordination function(DCF) inter-frame space (DIFS) and a contention window (CW) (e.g., CW=4)according to a random backoff, the first STA STA1 may acquire a transmitopportunity (TXOP) for transmitting a data frame 300. Here, the randombackoff denotes a procedure of additionally waiting for as much time asa product of a slot time (aSlotTime) and the CW.

The first STA STA1 may complete data transmission within the acquiredTXOP. For example, the first STA STA1 may transmit the data frame 300 toa second STA. When the data frame 300 is received, the second STA maytransmit an acknowledgement (ACK) frame 301 to the first STA STA1 ashort inter frame space (SIFS) after a time point at which reception ofthe data frame 300 is finished. When the ACK frame 301 is received, thefirst STA STA1 may determine that the data frame 300 has beensuccessfully received by the second STA. The first STA STA1 may transmita data frame 302 to the second STA2 an SIFS after a time point at whichreception of the ACK frame 301 is finished. When the data frame 302 isreceived, the second STA may transmit an ACK frame 303 to the first STASTA1 an SIFS after a time point at which reception of the data frame 302is finished. When the ACK frame 303 is received, the first STA STA1 maydetermine that the data frame 302 has been successfully received by thesecond STA.

When there is data to be additionally transmitted, the first STA STA1may acquire a TXOP again. The first STA STA1 may determine whether ornot the channel has been occupied based on CCA. When a result of CCAindicates that the channel is in the idle state, if the channel is inthe idle state within a DIFS and a CW (CW=2), the first STA STA1 mayacquire a TXOP for transmitting a data frame 304. The first STA STA1 maytransmit the data frame 304 to the second STA in the TXOP. When the dataframe 304 is received, the second STA may transmit an ACK frame 305 tothe first STA STA1 an SIFS after a time point at which reception of thedata frame 304 is finished.

When the ACK frame 305 is received, the first STA STA1 may determinethat the data frame 304 has been successfully received by the secondSTA. The first STA STA1 may transmit a data frame 306 to the second STA2an SIFS after a time point at which reception of the ACK frame 305 isfinished. When the data frame 306 is received, the second STA maytransmit an ACK frame 307 to the first STA STA1 an SIFS after a timepoint at which reception of the data frame 306 is finished. When the ACKframe 307 is received, the first STA STA1 may determine that the dataframe 306 has been successfully received by the second STA.

Every time the respective data frames 300, 302, 304, and 306 aretransmitted, the respective ACK frames 301, 303, 305, and 307 arerequired to be transmitted, and thus the overhead of the WLAN mayincrease due to the continuous transmissions of the ACK frames 301, 303,305, and 307. Also, MAC headers included in the respective data frames300, 302, 304, and 306 are identical to each other, and thus theoverhead of the WLAN may unnecessarily increase.

To solve these problems, an aggregate-MAC protocol data unit (A-MPDU)and an aggregate-MAC service data unit (A-MSDU) are defined in the IEEE801.11n standard.

FIG. 4 is a block diagram of a physical layer convergence procedure(PLCP) protocol data unit (PPDU) including an A-MPDU.

Referring to FIG. 4, a PPDU 400 may include a physical layer (PHY)header 410, a service field 420, and an A-MPDU 430. Also, the PPDU 400may further include at least one of a pad bit 440 and a tail bit 450.The A-MPDU 430 may include at least one MPDU 431 and 432. The first MPDU431 may include a MAC header 431-1 and an MSDU 431-2. The second MPDU432 may include a MAC header 432-1 and an MSDU 432-2. Using the PPDU 400including the A-MPDU 430, it is possible to minimize continuoustransmissions of ACK frames.

FIG. 5 is a block diagram of a PPDU including an A-MSDU.

Referring to FIG. 5, a PPDU 500 may include a PHY header 510, a servicefield 520, and an A-MSDU 530. Also, the PPDU 500 may further include atleast one of a pad bit 540 and a tail bit 550. The A-MSDU 530 mayinclude a MAC header 531 and at least one MSDU 532, 533, and 534. Usingthe PPDU 500 including the A-MSDU 530, it is possible to minimizecontinuous transmissions of ACK frames and MAC headers.

FIG. 6 is a conceptual diagram illustrating another example embodimentof a method of transmitting and receiving a frame in a WLAN.

Referring to FIG. 6, a first STA STA1 that intends to transmit data maydetermine whether or not a channel has been occupied based on CCA. Whena result of CCA indicates that the channel is in the idle state, if thechannel is in the idle state within a DIFS and a CW (CW=4), the firstSTA STA1 may acquire a TXOP for transmitting an A-MPDU 600 including atleast one MPDU. In other words, the first STA STA1 may transmit theA-MPDU 600 to a second STA in the TXOP. When the A-MPDU 600 is received,the second STA may transmit a block ACK (BA) frame 601 to the first STASTA1 an SIFS after a time point at which reception of the A-MPDU 600 isfinished. When the BA frame 601 is received, the first STA STA1 maydetermine that the A-MPDU 600 has been successfully received by thesecond STA.

When there is data to be additionally transmitted, the first STA STA1may acquire a TXOP again. In other words, the first STA STA1 maydetermine whether or not the channel has been occupied based on CCA.When a result of CCA indicates that the channel is in the idle state, ifthe channel is in the idle state within a DIFS and a CW (CW=2), thefirst STA STA1 may acquire a TXOP for transmitting an A-MSDU 602including at least one MSDU. In other words, the first STA STA1 maytransmit the A-MSDU 602 to the second STA in the TXOP. When the A-MSDU602 is received, the second STA may transmit a BA frame 603 to the firstSTA STA1 an SIFS after a time point at which reception of the A-MSDU 602is finished. When the BA frame 603 is received, the first STA STA1 maydetermine that the A-MSDU 602 has been successfully received by thesecond STA.

Using an A-MPDU or an A-MSDU, it is possible to minimize continuoustransmissions of ACK frames and MAC headers. However, since an A-MPDU oran A-MSDU is transmitted to only one STA, the use of an A-MPDU or anA-MSDU may be inefficient when many STAs are in one BSS. When data to betransmitted to a specific STA is very large, transmission of the datamay not be completed in a current TXOP. In this case, the data to betransmitted to the specific STA may be continuously transmitted in asubsequent TXOP after performing of CCA and a time corresponding to “aDIFS+a CW.” Likewise, data to be transmitted to another STA in the sameBSS may also be transmitted in the subsequent TXOP after performing ofCCA and the time corresponding to “the DIFS+ the CW.” In other words,when data to be transmitted is large or many STAs are in one BSS,temporal overhead may occur due to the time required for channel access(e.g., a DIFS and a CW). Therefore, the performance of a WLAN may bedegraded.

Further, the number of STAs associated to each BSS is drasticallyincreasing in a downtown area, and thus collisions between STAs in a BSSand collisions between STAs associated to neighboring BSSs areincreasing. In this situation, a transmission method for efficientlytransmitting data to many STAs associated to one BSS is necessary.

FIG. 7 is a sequence diagram illustrating a method of transmitting andreceiving a frame according to an example embodiment of presentinvention, and FIG. 8 is a flowchart illustrating an operation ofgenerating a PPDU in a method of transmitting and receiving a frameaccording to an example embodiment of present invention.

Referring to FIGS. 7 and 8, an AP may form a BSS. Respective terminalsSTA1 to STA4 may belong to the BSS and may be associated to the AP. TheAP that intends to transmit a first data unit DU1 to the first terminalSTA1 may determine whether or not a channel has been occupied based onCCA. When a result of CCA indicates that the channel is in the idlestate, if the channel is in the idle state within a DIFS and a CW, theAP may acquire a TXOP for transmitting the first data unit DU1 (S700).Here, the data unit may denote an MPDU or an MSDU.

After acquiring a TXOP, the AP may generate a PPDU including multiplexeddata units (S710). Specifically, the AP may determine whether or not adata unit to be transmitted to the terminals STA2 to STA4 other than thefirst terminal STA1 among the terminals STA1 to STA4 associated theretois in a transmission queue (S711). When any data unit to be transmittedto the other terminals STA2 to STA4 is not in the transmission queue,S720 may be performed as the subsequent operation.

On the other hand, when a data unit to be transmitted to the otherterminals STA2 to STA4 is in the transmission queue, the AP maymultiplex the data units in the frequency domain and generate a PPDUincluding the multiplexed data units (S712). The PPDU including the dataunits multiplexed in the frequency domain is as follows.

FIG. 9 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency domain.

Referring to FIG. 9, the PPDU may include PHY headers and payloads. TheAP may assign a first data unit DU1 to be transmitted to the firstterminal STA1 to a first subcarrier SC1 in a whole band, a second dataunit DU2 to be transmitted to the second terminal STA2 to a secondsubcarrier SC2 in the whole band, a third data unit DU3 to betransmitted to the third terminal STA3 to a fourth subcarrier SC4 in thewhole band, and a fourth data unit DU4 to be transmitted to the fourthterminal STA4 to a third subcarrier SC3 in the whole band. Here, toequalize the payload sizes of the respective subcarriers SC1 to SC4, theAP may add at least one of a pad bit (e.g., an MAC pad bit or a PHY padbit) and a tail bit to each of the data units DU1 to DU4.

Referring back to FIGS. 7 and 8, after multiplexing the data units inthe frequency domain, the AP may determine whether or not a data unit tobe transmitted to the terminals STA2 to STA4 other than the firstterminal STA1 among the terminals STA1 to STA4 associated thereto is inthe transmission queue (S713). When any data unit to be transmitted tothe other terminals STA2 to STA4 is not in the transmission queue, S720may be performed as the subsequent operation. In other words, the AP maytransmit the PPDU including the data units multiplexed in the frequencydomain.

On the other hand, when a data unit to be transmitted to the otherterminals STA2 to STA4 is in the transmission queue even if the dataunits are multiplexed in the frequency domain, the AP may multiplex thedata units in the frequency and time domains and generate a PPDUincluding the multiplexed data units (S714). The PPDU including the dataunits multiplexed in the frequency and time domains is as follows.

FIG. 10 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency and time domains.

Referring to FIG. 10, the PPDU may include PHY headers and payloads. TheAP may assign the first data unit DU1 to be transmitted to the firstterminal STA1, the second data unit DU2 to be transmitted to the secondterminal STA2, and the third data unit DU3 to be transmitted to thethird terminal STA3 to the first subcarrier SC1 in the whole band. TheAP may assign the second data unit DU2 to be transmitted to the secondterminal STA2 and the third data unit DU3 to be transmitted to the thirdterminal STA3 to the second subcarrier SC2 in the whole band. The AP mayassign the fourth data unit DU4 to be transmitted to the fourth terminalSTA4 and the first data unit DU1 to be transmitted to the first terminalSTA1 to the third subcarrier SC3 in the whole band. The AP may assignthe third data unit DU3 to be transmitted to the third terminal STA3 andthe fourth data unit DU4 to be transmitted to the fourth terminal STA4to the fourth subcarrier SC4 in the whole band.

Here, to equalize the payload sizes of the respective subcarriers SC1 toSC4, the AP may add at least one of a pad bit (e.g., an MAC pad bit or aPHY pad bit) and a tail bit to last data units DU1, DU3, and DU4included in the payloads.

Meanwhile, a PPDU including MPDUs multiplexed in the time domain and aPPDU including MSDUs multiplexed in the time domain are as follows.

FIG. 11 is a block diagram of an example embodiment of a PPDU includingMPDUs multiplexed in the time domain.

Referring to FIG. 11, a PPDU 1100 may include a PHY header 1101, aservice field 1102, an MPDU-1 1103, an MPDU-2 1104, and an MPDU-3 1105.Also, the PPDU 1100 may further include at least one of a pad bit 1106and a tail bit 1107. Each of MPDUs 1103, 1104, 1105 included in the PPDU1100 may denote a data unit of different user. For example, the MPDU-11103 may denote a data unit to be transmitted to the first terminalSTA1, the MPDU-2 1104 may denote a data unit to be transmitted to thesecond terminal STA2, and the MPDU-3 1105 may denote a data unit to betransmitted to the third terminal STA3.

FIG. 12 is a block diagram of an example embodiment of a PPDU includingMDSUs multiplexed in the time domain.

Referring to FIG. 12, a PPDU 1200 may include a PHY header 1201, aservice field 1202, a MAC header 1203, an MSDU-1 1204, an MSDU-2 1205,and an MSDU-3 1206. Also, the PPDU 1200 may further include at least oneof a pad bit 1207 and a tail bit 1208.

Each of MSDUs 1204, 1205, 1206 included in the PPDU 1200 may denote adata unit of different user. For example, the MSDU-1 1204 may denote adata unit to be transmitted to the first terminal STA1, the MSDU-2 1205may denote a data unit to be transmitted to the second terminal STA2,and the MSDU-3 1206 may denote a data unit to be transmitted to thethird terminal STA3.

Referring back to FIGS. 7 and 8, after multiplexing the data units inthe frequency and time domains, the AP may determine whether or not adata unit to be transmitted to the terminals STA2 to STA4 other than thefirst terminal STA1 among the terminals STA1 to STA4 associated theretois in the transmission queue (S715). When any data unit to betransmitted to the other terminals STA2 to STA4 is not in thetransmission queue, S720 may be performed as the subsequent operation.In other words, the AP may transmit the PPDU including the data unitsmultiplexed in the frequency and time domains.

On the other hand, when a data unit to be transmitted to the otherterminals STA2 to STA4 is in the transmission queue even if the dataunits are multiplexed in the frequency and time domains, the AP maymultiplex the data units in the frequency, time, and spatial domains andgenerate a PPDU including the multiplexed data units (S716). The PPDUincluding the data units multiplexed in the frequency, time, and spatialdomains is as follows.

FIG. 13 is a block diagram of an example embodiment of a PPDU includingdata units multiplexed in the frequency, time, and spatial domains.

Referring to FIG. 13, a PPDU may include PHY headers and payloads. Afirst space SP1 and a second space SP2 may be orthogonal to each other.The AP may assign data units to the first space SP1. In other words, theAP may assign the first data unit DU1 to be transmitted to the firstterminal STA1, the second data unit DU2 to be transmitted to the secondterminal STA2, and the third data unit DU3 to be transmitted to thethird terminal STA3 to the first subcarrier SC1 in the whole band. TheAP may assign the second data unit DU2 to be transmitted to the secondterminal STA2 and the third data unit DU3 to be transmitted to the thirdterminal STA3 to the second subcarrier SC2 in the whole band. The AP mayassign the fourth data unit DU4 to be transmitted to the fourth terminalSTA4 and the first data unit DU1 to be transmitted to the first terminalSTA1 to the third subcarrier SC3 in the whole band. The AP may assignthe third data unit DU3 to be transmitted to the third terminal STA3 andthe fourth data unit DU4 to be transmitted to the fourth terminal STA4to the fourth subcarrier SC4 in the whole band.

Also, the AP may assign data units to the second space SP2. The AP mayassign the first data unit DU1 to be transmitted to the first terminalSTA1, the second data unit DU2 to be transmitted to the second terminalSTA2, and the third data unit DU3 to be transmitted to the thirdterminal STA3 to the first subcarrier SC1 in the whole band. The AP mayassign the second data unit DU2 to be transmitted to the second terminalSTA2 and the third data unit DU3 to be transmitted to the third terminalSTA3 to the second subcarrier SC2 in the whole band. The AP may assignthe fourth data unit DU4 to be transmitted to the fourth terminal STA4and the first data unit DU1 to be transmitted to the first terminal STA1to the third subcarrier SC3 in the whole band. The AP may assign thethird data unit DU3 to be transmitted to the third terminal STA3 and thefourth data unit DU4 to be transmitted to the fourth terminal STA4 tothe fourth subcarrier SC4 in the whole band.

Here, to equalize the payload sizes of the respective subcarriers SC1 toSC4, the AP may add at least one of a pad bit (e.g., an MAC pad bit or aPHY pad bit) and a tail bit to last data units DU1, DU3, and DU4included in the payloads.

Meanwhile, the AP may generate a PPDU including data units multiplexedin the spatial domain for two major purposes. First, to acquire aspatial diversity gain, the AP may transmit a PPDU including the dataunits DU1 to DU4 multiplexed in the frequency and time domainsrepeatedly in the spatial domain. Second, to increase the transmissioncapacity, the AP may generate different PPDUs including the data unitsDU1 to DU4 multiplexed in the frequency and time domains and transmitthe different PPDUs in separate spaces.

Referring back to FIGS. 7 and 8, it has been described above thatfrequency multiplexing, frequency-time multiplexing, andfrequency-time-spatial multiplexing are sequentially performed accordingto the state of the transmission queue (e.g., the state of thetransmission queue after multiplexing) in operation S710. In addition tothis, in another example embodiment of the present invention, the AP maygenerate a PPDU based on at least one of frequency multiplexing, timemultiplexing, and spatial multiplexing according to the state of thetransmission queue. In other words, the AP may generate a PPDU includingdata units multiplexed based on the size of data currently stored in thetransmission queue without considering the state of the transmissionqueue after multiplexing. For example, when many data units arecurrently stored in the transmission queue, the AP may generate a PPDUincluding data units multiplexed in the frequency, time, and spatialdomains. On the other hand, when few data units are currently stored inthe transmission queue, the AP may generate a PPDU including data unitsmultiplexed in the frequency, time, or spatial domain.

Meanwhile, a signal (SIG) field of the PPDU may include at least one ofthe lengths of the data units, the receiver identifiers (IDs) of thedata units (e.g., association IDs (AIDs), partial AIDs (PAIDs), andgroup IDs), channels in which the data units are transmitted,transmission sections, and transmission spaces. Here, the SIG field maydenote a high throughput (HT)-SIG A field or an HT-SIG B field definedin the IEEE 802.11n standard. Alternatively, the SIG field may denote avery high throughput (VHT)-SIG A field or a VHT-SIG B field defined inthe IEEE 802.11ac standard. For example, the receiver IDs of the dataunits may be set in a group ID field included in the VHT-SIG A field.The spaces in which the data units are transmitted may be set in anumber of spatial-time stream (NSTS) field included in the VHT-SIG Afield.

The AP may generate the PPDU and then transmit the PPDU to the terminalsSTA1 to STA4 (S720). When the PPDU is received, the respective terminalsSTA1 to STA4 may acquire the SIG field included in the PPDU (S730-1,S730-2, S730-3, and S730-4). Based on information included in the SIGfield, each of the respective terminals STA1 to STA4 may determinewhether or not a data unit for the terminal is in the PPDU. When a dataunit for the terminal is in the PPDU, the terminal may determineresources (i.e., frequency resources, time resources, or spaceresources) through which the data unit for the terminal is transmittedbased on the information included in the SIG field. The respectiveterminals STA1 to STA4 may acquire data units through resourcesindicated by the SIG field (S740-1, S740-2, S740-3, and S740-4). Whenthe corresponding data unit is successfully received, each of theterminals STA1 to STA4 may transmit an ACK frame to the AP an SIFS aftera time point at which the PPDU is received (S750).

At this time, each of the terminals STA1 to STA4 may transmit a BA frameto the AP as a response to the reception of the corresponding data unit.Here, each of the terminals STA1 to STA4 may transmit the correspondingACK frame to the AP through resources corresponding to resources throughwhich a data unit for the terminal has been transmitted. For example,when a data unit is received through a second transmission section in afirst subcarrier of a PPDU received from a fourth antenna, each of theterminals STA1 to STA4 may set an ACK frame which is a response to thedata unit in a second transmission section in a first subcarrier of aPPDU to be transmitted through the fourth antenna.

FIG. 14 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency domain. Referring to FIG. 14, an AP that intends totransmit a first data unit DU1 for a first terminal STA1 may determinewhether or not a channel has been occupied based on CCA. When a resultof CCA indicates that the channel is in the idle state, if the channelis in the idle state within a DIFS and a CW (CW=4), the AP may acquire aTXOP. The AP may determine whether or not a data unit to be transmittedto terminals STA2 to STA4 other than the first terminal STA1 among theterminals STA1 to STA4 associated thereto is in a transmission queue.

When a data unit to be transmitted to the other terminals STA2 to STA4is in the transmission queue, the AP may multiplex data units DU1 to DU4for the respective terminals STA1 to STA4 in the frequency domain. Forexample, the first data unit DU1 for the first terminal STA1 may beassigned to a first subcarrier SC1, the second data unit DU2 for thesecond terminal STA2 may be assigned to a second subcarrier SC2, thefourth data unit DU4 for the fourth terminal STA4 may be assigned to athird subcarrier SC3, and the third data unit DU3 for the third terminalSTA3 may be assigned to a fourth subcarrier SC4.

The AP may transmit a PPDU including the data units DU1 to DU4multiplexed in the frequency domain to the terminals STA1 to STA4. Therespective terminals STA1 to STA4 may receive the PPDU, and determinewhether or not the data units DU1 to DU4 thereof are in the PPDU basedon information included in the SIG field of the PPDU. When the dataunits DU1 to DU4 thereof are in the PPDU, the respective terminals STA1to STA4 may determine resources through which the data units DU1 to DU4thereof are transmitted based on the information included in the SIGfield of the PPDU.

The respective terminals STA1 to STA4 may receive the data units DU1 toDU4 thereof through the resources indicated by the SIG field of thePPDU. When the data units DU1 to DU4 are successfully received, therespective terminals STA1 to STA4 may transmit ACK frames to the AP anSIFS after a time point at which reception of the PPDU is finished. Atthis time, the respective terminals STA1 to STA4 may transmit the ACKframes to the AP through resources corresponding to the resourcesthrough which the data units DU1 to DU4 thereof have been transmitted.

FIG. 15 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency and time domains.

Referring to FIG. 15, an AP that intends to transmit a first data unitDU1 for a first terminal STA1 may determine whether or not a channel hasbeen occupied based on CCA. When a result of CCA indicates that thechannel is in the idle state, if the channel is in the idle state withina DIFS and a CW (CW=4), the AP may acquire a TXOP. The AP may determinewhether or not a data unit to be transmitted to terminals STA2 to STA4other than the first terminal STA1 among the terminals STA1 to STA4associated thereto is in a transmission queue.

When a data unit to be transmitted to the other terminals STA2 to STA4is in the transmission queue, the AP may multiplex data units DU1 to DU4for the respective terminals STA1 to STA4 in the frequency and timedomains. For example, the first data unit DU1 for the first terminalSTA1, the second data unit DU2 for the second terminal STA2, and thethird data unit DU3 for the third terminal STA3 may be assigned to afirst subcarrier SC1. The second data unit DU2 for the second terminalSTA2 and the third data unit DU3 for the third terminal STA3 may beassigned to a second subcarrier SC2. The fourth data unit DU4 for thefourth terminal STA4 and the first data unit DU1 for the first terminalSTA1 may be assigned to a third subcarrier SC3. The third data unit DU3for the third terminal STA3 and the fourth data unit DU4 for the fourthterminal STA4 may be assigned to a fourth subcarrier SC4.

The AP may transmit a PPDU including the data units DU1 to DU4multiplexed in the frequency and time domains to the terminals STA1 toSTA4. The respective terminals STA1 to STA4 may receive the PPDU, anddetermine whether or not the data units DU1 to DU4 thereof are in thePPDU based on information included in the SIG field of the PPDU. Whenthe data units DU1 to DU4 thereof are in the PPDU, the respectiveterminals STA1 to STA4 may determine resources through which the dataunits DU1 to DU4 thereof are transmitted based on the informationincluded in the SIG field of the PPDU.

The respective terminals STA1 to STA4 may receive the data units DU1 toDU4 thereof through the resources indicated by the SIG field of thePPDU. When the data units DU1 to DU4 are successfully received, therespective terminals STA1 to STA4 may transmit ACK frames to the AP anSIFS after a time point at which reception of the PPDU is finished. Atthis time, the respective terminals STA1 to STA4 may transmit the ACKframes to the AP through resources corresponding to the resourcesthrough which the data units DU1 to DU4 thereof have been transmitted.

FIG. 16 is a conceptual diagram of an example embodiment of a method oftransmitting and receiving a PPDU including data units multiplexed inthe frequency, time, and spatial domains.

Referring to FIG. 16, an AP that intends to transmit a first data unitDU1 for a first terminal STA1 may determine whether or not a channel hasbeen occupied based on CCA. When a result of CCA indicates that thechannel is in the idle state, if the channel is in the idle state withina DIFS and a CW (CW=4), the AP may acquire a TXOP. The AP may determinewhether or not a data unit to be transmitted to terminals STA2 to STA4other than the first terminal STA1 among the terminals STA1 to STA4associated thereto is in a transmission queue.

When a data unit to be transmitted to the other terminals STA2 to STA4is in the transmission queue, the AP may multiplex data units DU1 to DU4for the respective terminals STA1 to STA4 in the frequency, time, andspatial domains. For example, the first data unit DU1 for the firstterminal STA1, the second data unit DU2 for the second terminal STA2,and the third data unit DU3 for the third terminal STA3 may be assignedto a first subcarrier SC1 in a first space SP1. The second data unit DU2for the second terminal STA2 and the third data unit DU3 for the thirdterminal STA3 may be assigned to a second subcarrier SC2 in the firstspace SP1. The fourth data unit DU4 for the fourth terminal STA4 and thefirst data unit DU1 for the first terminal STA1 may be assigned to athird subcarrier SC3 in the first space SP1. The third data unit DU3 forthe third terminal STA3 and the fourth data unit DU4 for the fourthterminal STA4 may be assigned to a fourth subcarrier SC4 in the firstspace SP1.

Also, the first data unit DU1 for the first terminal STA1, the seconddata unit DU2 for the second terminal STA2, and the third data unit DU3for the third terminal STA3 may be assigned to a first subcarrier SC1 ina second space SP2. The second data unit DU2 for the second terminalSTA2 and the third data unit DU3 for the third terminal STA3 may beassigned to a second subcarrier SC2 in the second space SP2. The fourthdata unit DU4 for the fourth terminal STA4 and the first data unit DU1for the first terminal STA1 may be assigned to a third subcarrier SC3 inthe second space SP2. The third data unit DU3 for the third terminalSTA3 and the fourth data unit DU4 for the fourth terminal STA4 may beassigned to a fourth subcarrier SC4 in the second space SP2.

The AP may transmit a PPDU including the data units DU1 to DU4multiplexed in the frequency, time, and spatial domains to the terminalsSTA1 to STA4. The respective terminals STA1 to STA4 may receive thePPDU, and determine whether or not the data units DU1 to DU4 thereof arein the PPDU based on information included in the SIG field of the PPDU.When the data units DU1 to DU4 thereof are in the PPDU, the respectiveterminals STA1 to STA4 may determine resources through which the dataunits DU1 to DU4 thereof are transmitted based on the informationincluded in the SIG field of the PPDU.

The respective terminals STA1 to STA4 may receive the data units DU1 toDU4 thereof through the resources indicated by the SIG field of thePPDU. When the data units DU1 to DU4 are successfully received, therespective terminals STA1 to STA4 may transmit ACK frames to the AP anSIFS after a time point at which reception of the PPDU is finished. Atthis time, the respective terminals STA1 to STA4 may transmit the ACKframes to the AP through resources corresponding to the resourcesthrough which the data units DU1 to DU4 thereof have been transmitted.

FIG. 17 is a block diagram showing the constitution of an AP accordingto an example embodiment of the present invention.

Referring to FIG. 17, an AP 1700 may include a channel access unit 1701,a transmission queue check unit 1702, a signal transmission unit 1703, aband division unit 1704, a frame combination unit 1705, and an antennadisposition unit 1706. The processor 110 described above with referenceto FIG. 1 may perform functions of the channel access unit 1701, thetransmission queue check unit 1702, the signal transmission unit 1703,the band division unit 1704, the frame combination unit 1705, and theantenna disposition unit 1706.

The channel access unit 1701 may attempt channel access based on the DCFor enhanced distributed channel access (EDCA). In this case, a STA thatwill access a channel may be determined based on an access category (AC)of a data unit, the size of a CW, and so on. When a terminal is causedto access a channel by the channel access unit 1701, the transmissionqueue check unit 1702 may determine whether a data unit for a terminalother than the terminal currently accessing the channel is in atransmission queue. When any data unit for a terminal other than theterminal currently accessing the channel is not in the transmissionqueue, the signal transmission unit 1703 may transmit a PPDU includingonly data units for the terminal currently accessing the channel.

On the other hand, when a data unit for a terminal other than theterminal currently accessing the channel is in the transmission queue,the band division unit 1704 may multiplex data units for the terminalsin the frequency domain. Subsequently, the transmission queue check unit1702 may determine whether a data unit for a terminal other than theterminal currently accessing the channel is in the transmission queue.When any data unit for a terminal other than the terminal currentlyaccessing the channel is not in the transmission queue, the signaltransmission unit 1703 may transmit a PPDU including the data unitsmultiplexed in the frequency domain.

On the other hand, when a data unit for a terminal other than theterminal currently accessing the channel is in the transmission queue,the band division unit 1704 and the frame combination unit 1705 maymultiplex data units for the terminals in the frequency and timedomains. In other words, the frame combination unit 1705 may reconfigurea frame by adding the data unit for the other terminal to each of thesubcarriers. Subsequently, the transmission queue check unit 1702 maydetermine whether a data unit for a terminal other than the terminalcurrently accessing the channel is in the transmission queue. When anydata unit for a terminal other than the terminal currently accessing thechannel is not in the transmission queue, the signal transmission unit1703 may transmit a PPDU including the data units multiplexed in thefrequency and time domains.

On the other hand, when a data unit for a terminal other than theterminal currently accessing the channel is in the transmission queue,the band division unit 1704, the frame combination unit 1705, and theantenna disposition unit 1706 may multiplex data units for the terminalsin the frequency, time, and spatial domains. In other words, the antennadisposition unit 1706 may reconfigure a frame by multiplexing the dataunits for the terminals in the spatial domain. The signal transmissionunit 1703 may transmit a PPDU including the data units multiplexed inthe frequency, time, and spatial domains.

Thus far, methods of multiplexing data units in the time, frequency, andspatial domains have been described in detail. In an existing WLAN, onlyone STA occupying a channel exclusively uses the whole channel during awhole TXOP, and other STAs are required to wait during the TXOP. Also,since many STAs attempt channel access after the TXOP, the problem of achannel access delay continues. To solve these problems, the methodsaccording to example embodiments of the present invention have beenproposed.

According to example embodiments of the present invention, many STAs inone BBS may be frequently given a TXOP. For this reason, a time to waitfor data transmission may be reduced, and the performance of a WLAN(e.g., communication quality and transmission stability) may beimproved.

Example embodiments of the present invention may be implemented in theform of program instructions executable through various computer meansand recorded in a computer-readable medium. The computer-readable mediummay include program instructions, data files, data structures, etc.,separately or in combination. The program instructions recorded in thecomputer-readable medium may be specially designed and formed for theexample embodiments of the present invention, or may be known to andused by those of ordinary skill in the art of the computer softwarefield.

The computer-readable medium may be a hardware device speciallyconfigured to store and execute program instructions, such as a ROM, aRAM, or a flash memory. The hardware device may be configured to operateas at least one software module to perform the operation according toexample embodiments of the present invention, and vice versa. Theprogram instruction may be mechanical codes as made by a compiler, aswell as high-level language codes executable by a computer based on aninterpreter or the like.

While the example embodiments of the present invention and theiradvantages have been described in detail, it should be understood thatvarious changes, substitutions and alterations may be made hereinwithout departing from the scope of the invention.

What is claimed is:
 1. An operation method performed in an access point(AP), the operation method comprising: generating a physical layerconvergence procedure (PLCP) protocol data unit (PPDU) including aplurality of data units; and transmitting the PPDU based on anorthogonal frequency division multiple access (OFDMA) manner, whereinthe plurality of data units includes a first data unit for a firststation and a second data unit for a second station, the first data unitand the second data unit are multiplexed in a frequency domain, apadding is appended to each of the first data unit and the second dataunit so that a length of a first payload including the first data unitis configured to be identical with a length of a second payloadincluding the second data unit, the PPDU includes a signal (SIG) field,and the SIG field includes first identifier of the first station and asecond identifier of the second station.